1. Field of the Invention
The present invention relates to a method of setting power for a reproducing beam to be applied to a magneto-optical disc at a constant angular velocity to reproduce data signals therefrom in the magnetically induced super resolution mode. The invention also relates to a method of reproducing data signals from a magneto-optical disc by applying a beam of the power thus set, to the magneto-optical disc.
2. Related Art
Magneto-optical discs are known as recording media on which data signals can be rewritten. A magneto-optical disc comprises a substrate and a signal-recording layer provided on the substrate. The signal-recording layer has recording tracks on its surface. A recording beam may be applied to the signal-recording layer while the magneto-optical disc is rotating, thereby to record data signals along the recording tracks. A reproducing beam may be applied to the signal-recording layer while the magneto-optical disc is rotating, thereby to reproduce data signals from the recording tracks. Data can be recorded on the magneto-optical disc in two modes, which are known as xe2x80x9clight modulation modexe2x80x9d and xe2x80x9cmagnetic-field modulation mode,xe2x80x9d respectively.
In the light modulation mode, an external magnetic field generated from a prescribed bias is applied to the signal-recording layer that has been magnetized in a predetermined direction. A laser beam modulated in accordance with a data signal to be recorded is applied to the signal-recording layer thus magnetized. Record marks are thereby formed on those parts of the signal-recording layer which have been irradiated with the laser beam, whereby data is recorded on the magneto-optical disc. Note that the record marks are magnetized in the direction of the external magnetic field.
To reproduce data from a magneto-optical disc, a laser beam less intense than the beam applied to record the data on the disc is applied to the recording track of the disc, and the bean reflected from the signal-recording layer of the disc is detected. The plane of polarization of the laser beam rotates in the signal-recording layer by virtue of the so-called magnetic Kerr effect. Hence, any record mark on the disc is detected from the difference in intensity between the beam reflected from the record mark and the beam reflected from any other part of the signal-recording layer. The data is thereby reproduced from the magneto-optical disc.
In recent years, it has been increasingly demanded that recording media be smaller and store more data. Recording density should therefore be increased in magneto-optical discs, too. With the conventional magneto-optical disc it is easy to form smaller record marks by controlling the intensity of the laser beam. It is difficult, however, to detect small record marks at high S/N ratio.
To overcome the limitation to the data reproduction from the conventional magneto-optical disc, it is proposed that data be recorded and reproduced by means of the technique called xe2x80x9cmagnetically induced super resolution (MSR).xe2x80x9d This technique uses a magneto-optical disc with a signal-recording layer composed of a plurality of magnetic layers the magnetic characteristic of which changes with temperature. With the disc of this type it is possible to reproduce data from record marks that are smaller than the spot the laser beam forms on the signal-recording layer 2. Various proposals have been proposed about the multi-layer structure of the signal-recording layer and the method of reproducing data from record marks.
In the case of the conventional 5.25-inch magneto-optical disc, the user area, in and from which data signals are recorded and reproduced, is a region that has inner and outer diameters of 30 mm and 60 mm, respectively. The 5.25-inch magneto-optical disc is rotated at a constant angular velocity (CAV) in the process of recording or reproducing data. Thus, the linear velocity at the outermost track of the user area is 2xc2xd times, or about 1.4 times, as high as that at the innermost track.
While the magneto-optical disc is rotating in the process of recording data on it or reproducing data from it, it is supported at its center in the magneto-optical disc drive. Thus supported, the disc may warp due to its own weight, assuming a shape like a bowl.
To reproduce data from the magneto-optical disc by means of magnetically induced super resolution, it is necessary to control the intensity of the laser beam at high precision to attain the MSR effect as is desired. This is because the magnetic characteristic of the signal-recording layer greatly depends on temperature as has been mentioned above. In particular, the intensity of the beam must be increased with high precision to reproduce data while rotating the disc at a constant angular velocity. Since the linear velocity is higher at any track than at an inner track, the beam applied to a track should be more intense than the one applied to any inner track.
Therefore, the intensity of the reproducing beam is linearly increased as the beam spot moves from the innermost track to the outermost track in the user area of the magneto-optical disc. The beam applied to the outermost track is, for example, 2xc2xd times as high as the bean applied to the innermost track.
However, the magneto-optical disc warps in the process of reproducing data from it, as has been described above. The disc inclines more at a track than at any inner track. Namely, the signal-recording layer inclines to the laser beam applied to the disc, at an increasing angle toward the outermost recording track. The spot the laser beam forms on the signal-recording layer inevitably has coma-aberration. Moreover, the beam spot is de-focused. Consequently, no sufficient temperature rise can take place at that part of the signal-recording layer which is irradiated with the laser beam. This makes it impossible to attain high or stable MSR effect.
Accordingly, an object of the present invention is to provide a method of setting intensity for a reproducing beam applied to a rotating magneto-optical disc to reproduce data therefrom by means of the MSR technique, thereby to heat the signal-recording layer of the disc sufficiently even if the disc warps and attain an adequate MSR effect. Another object of the invention is to provide a method of reproducing data from a magneto-optical disc by applying a beam of the intensity thus set, to the magneto-optical disc.
According to the invention, there is provided a method of setting power for a reproducing beam to be applied to a magneto-optical disc to reproduce data signals therefrom in magnetically induced super resolution mode, while the disc is rotating at a constant angular velocity. The method comprises: a first calibration step of performing calibration on a signal-recording region of the magneto-optical disc, at least two positions on a radius of the magneto-optical disc, thereby to calculate optimal data-reproducing powers for said at least two positions; and a second calibration step of setting a data-reproducing power optimal for a given position on the radius of the magneto-optical disc, by performing curvilinear approximation on the data-reproducing powers calculated in the first calibration step.
According to the invention, there is provided a method of reproducing data signals from a magneto-optical disc, while the disc is rotating at a constant angular velocity. The method comprises: a first calibration step of performing calibration on a signal-recording region of the magneto-optical disc, at at least two positions on a radius of the magneto-optical disc, thereby to calculate optimal data-reproducing powers for said at least two positions; a second calibration step of setting a data-reproducing power optimal for a given position on the radius of the magneto-optical disc, by performing curvilinear approximation on the data-reproducing powers calculated in the first calibration step; and a data-reproducing step of applying a laser beam to the magneto-optical disc, thereby to reproduce data signals from the disc, said laser beam having intensity that corresponds to the data-reproducing power set in the second calibration step.
In the method of setting power for a reproducing beam to be applied to a magneto-optical disc to reproduce data therefrom, and the method of reproducing data from a magneto-optical disc, power optimal for the reproducing beam is obtained by curvilinear approximation based on the powers calculated by calibration performed at at least two points on the radius of the disc. The signal-recording layer of the disc can thereby be sufficiently heated even if the disc warps, causing coma aberration of the beam spot and de-focusing of the reproducing beam. Therefore, a stable MSR effect can be obtained.
In the method and apparatus according to the invention, power optimal for a reproducing beam to be applied at a position on a magneto-optical disc is obtained by curvilinear approximation based on the powers calculated by calibration performed at least two points on the radius of the disc. The signal-recording layer of the disc can thereby be sufficiently heated even if the disc warps, causing coma aberration of the beam spot and de-focusing of the reproducing beam. Therefore, a stable MSR effect can be obtained. Hence, the margin of power for the reproducing beam can be increased, whereby data can be reliably reproduced in the MSR mode from the magneto-optical disc, though the magnetic characteristic of the signal-recording layer greatly depends on temperature.